Tokyo: A 20-centimetre tsunami has hit Japan after a powerful and shallow undersea quake, broadcaster NHK said, although there were no immediate signs of serious damage or injury.

People were being warned to stay away from the coast with the small wave rolling ashore in Ishinomaki around an hour after the 7.1 magnitude quake.

Workers at the crippled Fukushima nuclear plant were "ordered to evacuate from the waterfront", Kyodo reported, also quoting the Tokyo Electric Power Company (TEPCO) as saying no new abnormalities had been found at the power station.

The quake struck at a depth of 10km at 2.10am local time, 327km southeast of Ishinomaki in Miyagi Prefecture, according to the US Geological Survey.

The USGS initially said its magnitude was 7.5, and issued a green alert on its website, signalling a low probability of deaths or economic losses.

Fairfax journalist Jared Lynch, who was in Tokyo, said he was woken by the quake.

"It felt like a tram was rolling past our room. The bed shook a little and it lasted for about 30 seconds," he said. "But we are safe and everything seems like business as usual."

Sydney resident John Orr, who was awake at the time of the quake, said it caused his room on the seventh floor of a hotel in Tokyo to "sway significantly".

"I didn't time it but I guess it lasted for about 30 seconds," Mr Orr said. 

In the town of Ofunato, a 20-centimetre tsunami was logged just after 3am. The waves were expected to hit several places along the northeast coast that was devastated by the huge tsunami of 2011.

NHK was telling viewers in the area that there remained a possibility that secondary waves could strike, but live footage from harbours in several towns showed no significant changes.

Eastern Japan, a seismically active region, was struck by a 6.5 magnitude earthquake last month causing tremors that were felt 600 kilometres away in Tokyo.

More than 18,000 people died when a 9.0 magnitude sub-sea earthquake sent a towering tsunami barrelling into Japan’s northeast coast in March 2011 in the country’s worst post-World War II disaster.

Cooling systems at the Fukushima nuclear plant were knocked out, sending reactors into meltdown and forcing tens of thousands of people to flee.

AFP with Jared Lynch